Methods and systems of testing formation samples using a rock hydrostatic compression chamber

ABSTRACT

A micro-CT sample holder for placing a sample under hydrostatic pressure during scanning, a micro-CT scanning system using the sample holder, and method of rock sample inspection under hydrostatic pressure. The sample holder includes a pressure vessel having a thinned-walled region and an interior chamber fixed reference stops inside the interior chamber. A position-locating anvil holds the sample and rests on the one or more fixed reference stops so the position of the sample is fixed under pressure. The thin-walled region surrounds the sample to minimize radiation interactions yet sufficient for maintaining a design pressure within the pressure vessel. The pressure vessel includes threads near the top opening to receive a compression screw assembly for closing the top opening and applying a variable pressure on the pressure vessel with a piston acting on a fluid inside the interior chamber.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority to U.S. Pat. App. 62/106,712titled “Methods and Systems of Testing Formation Samples Using a RockCompression Chamber”, filed Jan. 22, 2015 by inventor Abraham Grader,which is incorporated by reference in its entirety.

BACKGROUND

Oilfield operators spend a great deal of time and resources whendrilling and developing fields for petroleum products. It is essentialfor the operators to obtain detailed rock properties in order tooptimize the production process. Some existing techniques fordetermining rock properties are not effective for many types of rocks.

BRIEF DESCRIPTION OF THE DRAWINGS

Accordingly, there are disclosed in the drawings and the followingdescription methods and systems of testing formation samples using ahydrostatic rock compression chamber:

FIG. 1 is a cross-sectional diagram of an illustrative rock compressionchamber in a micro-CT configuration; and

FIG. 2 is a line drawing of a preferred embodiment of the samplepositioning in the illustrative rock compression chamber.

It should be understood, however, that the specific embodiments given inthe drawings and detailed description thereto do not limit thedisclosure. On the contrary, they provide the foundation for one ofordinary skill to discern the alternative forms, equivalents, andmodifications that are encompassed together with one or more of thegiven embodiments in the scope of the appended claims.

DETAILED DESCRIPTION

Disclosed herein are methods and systems for testing formation samplesusing a hydrostatic rock compression chamber (sample holder) to inspecta rock sample from an underground formation suspected of containing oiland gas products but not limited to oil and gas. In this disclosure, thesample may be (but not limited to) a cylinder of formation rock that issuspected to contain fluids in the form of water, oil, or gas in unknownconcentrations. The sample may be imaged using X-ray CT prior tocompression to determine its interior solid and pore structure fromwhich various properties are determined. Applying pressure to the rocksample is essential to place the sample under simulated conditionsdownhole and to witness and record the elastic and plastic recovery ofthe rock under simulated downhole stress conditions. The images of therock under stress yield, through analysis, the rock properties and theirdependency on stress.

Accordingly, FIG. 1 shows an illustrative rock compression chamber testassembly 100 (test assembly including the sample holder) which iscomprised of a relief valve 102, a pressure gauge 104, a compressionscrew assembly 106, a vessel body assembly 108, an optional strain gauge110 with a set of strain gauge wires 112, a rock sample assembly 114, abottom plug 116, and an optional plastic enforcer tube 118. The straingauge 110 may be connected to a monitoring device (not shown) by thestrain gauge wires 112 to monitor and record pressures felt by the testassembly 100. A test measurement device, such as a micro-CT scanner,comprised of a radiation source 180A, a detector 180B, along with acontroller and measurement recording device 190 is used to irradiate thevessel body assembly 108 (and the sample enclosed within) with radiationand record the resultant signals. The optional plastic enforcer tube 118may be used to add structural strength to the vessel body assembly 108,specifically around a thin-walled region 170. The plastic enforcer tube118 is optional, but when used may prevent bending when the thin-walledregion 170 is of such length that the thinness results in a lowstructural strength despite being sufficient to maintain the designpressure levels internally.

The vessel body assembly 108 is comprised of a vessel body 160, a set ofthreads 162, and an interior chamber 168 including an air chamber 164and a hydraulic fluid chamber 166. The interior chamber 168 extends mostof the length of the interior of the vessel body 160 and includes theair chamber 164. The vessel body assembly 108 is made of titanium alloyor other similar material that has high strength while having as littlemass as possible. The use of titanium or similar materials is preferredto minimize the losses associated with interactions with theradiation(s) emitted by the source 180A. In one embodiment, the interiorchamber 168 is cylindrical with an inner diameter of 7 mm, with the airchamber 164 cylindrical with an inner diameter of 6 mm. Otherembodiments have interior diameters of 1-5 mm. Typical thickness of thewall in the thin-walled region is 1 mm of titanium alloy but may be asthin as 0.5 mm in other embodiments, based on the design pressure. Atypical design pressure is 4000 psig with operational pressures up tothe 2500 psig range. All embodiments include appropriate safety factorsfor any given component.

Continuing with the vessel body assembly 108, the bottom plug 116 isplaced into the interior chamber 168. The bottom plug 116 uses at leastone O-ring 150 to create a fluid-tight seal to define and isolate theair chamber 164. The vessel body 160 also includes the thin-walledregion 170 to minimize the amount of matter the radiation has totraverse when emitted from the source 180A to the detector 180B.

The compression screw assembly 106 is comprised of a compression screwbody 140 (stainless steel 316 or similar alloy for materialcompatibility with the vessel body 160), a conduit connector 142, a setof threads 144 for mechanical coupling to the threads 162 present on thevessel body 160, a piston arm 146, a fluid conduit 148 located withinthe piston arm 146, and at least one O-ring 150 to maintain pressure andestablish fluid isolation when the test assembly 100 is in use. Thecompression screw assembly 106 may be made of steel or other materialthat can withstand high pressures without deforming. Reference marks(not shown) may be placed on the outside of the compression screwassembly 106 to allow an operator to monitor the position of thecompression screw assembly 106 in reference to the vessel body assembly108. The conduit connector 142 may either be used to seal off the fluidconduit 148 or to attach additional test devices such as the reliefvalve 102 or the pressure gauge 104 as required. Once screwed into thevessel body assembly 108, the compression screw assembly 106 may betightened using the threads 144 against the vessel body 160 threads 162.In this way, the piston arm 146 of the compression screw assembly 106reduces the volume of the interior chamber 168 of the vessel bodyassembly 108. As a consequence, as the volume of the interior chamber168 is reduced, the pressure in the interior chamber 168 is increased.Pressure in the interior chamber 168 is thus controlled by tightening orloosening the compression screw assembly 106 in relation to the vesselbody assembly 108. Pressure felt at the end of the piston arm 146 may bemonitored by attaching the pressure gauge 104 to the fluid conduit 148.

The hydraulic fluid is preferably only slightly compressible (less than1% at 1000-4000 psig at room temperature is standard) so that thecompression of the air chamber allows for ease of setting the pressureinside the interior chamber 168. In an embodiment without the airchamber 164, the hydraulic fluid is preferably more compressible thanstandard as standard hydraulic fluid acted on by a piston makes settinga precise pressure level difficult.

To conduct an analysis, the sample is prepared. A sample of formationrock of interest (not shown) may be prepared by cutting a sample of rockinto a cylinder shape approximately 5 mm long and 5 mm in diameter. Therock sample is then preferably encapsulated by a covering that isimpermeable to fluids such as water and petroleum components. Thecovering may be one of heat shrink material, waterproof paint, plasticwrap, or any of a number of other materials. The purpose of the coveringseparate the rock from the compressing fluid in the chamber 166, so thatnet confining stress is transmitted to the rock sample assembly 114. Therock sample assembly 114 is thus comprised of a portion of formationrock cut into a cylinder shape and covered in a fluid-tight covering. Inother embodiments, the rock sample assembly includes one or two anvils(see FIG. 2) either below or both above and below the rock sampleitself. Preferably, the anvils are made of aluminum or other materialsimilar in composition to the rock sample so as not to interfere withthe sample testing.

The threads 162 are shown internal, but other embodiments may beexternal, so long as the compression screw assembly 106 has matchingthreads and the piston engages the hydraulic fluid in the hydraulicfluid chamber 116. Note that the base 172 may include screws or dowels(not shown) or holes to accept screws or dowels to aid in placement andin securing the sample holder in the micro-CT scanner.

Turning now to FIG. 2, the line drawing of a preferred embodiment of thesample positioning in the illustrative rock compression chamber isshown. In this embodiment, the rock sample assembly 114 includes therock sample 210 between a position-locating anvil 220 below the rocksample 210 and an upper anvil 230 above the rock sample. Theposition-locating anvil 220 rests on shoulders 250 that act as fixedreference stops 250. The user knows that the position-locating anvil 220will rest on the fixed reference stops 250 and that the bottom of thesample 210 meets the top of the position-locating anvil 220. So thelocation of the bottom of the sample 210 is always known and fixed. Theupper anvil 230 is optional.

Using the upper anvil 230 allows for encapsulation to occur part of allof the upper anvil 230, the sample 210, and all or part of theposition-locating anvil 220. The position-locating anvil 220 preferablyhas passages 225 for the hydraulic fluid to flow below theposition-locating anvil 220 and contact the bottom plug 116. The bottomplug 116 is sized to fit snugly against the inner wall so that a gasbubble of varying size is maintained in the air chamber 164. Inpractice, the bottom plug 116 may use the O-ring 150 shown in FIG. 1 orbe properly sized for an inside wall sufficiently uniform in geometrythat capillary forces prevent the hydraulic fluid from invading the airchamber 164. The shoulder 250 may be completely perpendicular to theinner wall or have a slope. In another embodiment, the stop or stops 250may be part of an insert that bottom plug 116 moves along or insideinstead of being integral with the vessel body 160, so long as the airchamber 164 is not invaded by the hydraulic fluid to prevent control ofthe pressure within the interior chamber 168 within the hydraulic fluidchamber 166.

To prepare the test assembly 100 for testing, the vessel body assembly108 is cleaned of all contaminants. The bottom plug 116 is inserted intothe interior chamber 168. It is important to not allow the bottom plug116 to travel all of the way to the bottom of the vessel body assembly108 as the air chamber 164 plays an important part in the test assembly100. Then, the rock sample assembly 114 is placed in the interiorchamber 168, resting above the bottom plug 116. Hydraulic fluid withinhydraulic fluid chamber 166, surrounding the encapsulated sampleassembly 114, is then injected into the interior chamber 168 above therock sample assembly 114, bottom plug 116, and the air chamber 164filling the remainder of the interior chamber 168 with fluid to form thehydraulic fluid chamber 166 around the rock sample assembly 114. Thusassembled, the interior chamber 168 contains the rock sample assembly114 above the air chamber 164 and surrounded by hydraulic fluid in thehydraulic fluid chamber 166. Finally, the compression screw assembly 106is threaded into the vessel body assembly 108.

To conduct an analysis of the rock sample assembly 114, the compressionscrew assembly 106 is tightened in relation to the vessel body assembly108 while the pressure in the interior chamber 168 is monitored by thepressure gauge 104 or the strain gauge 110. It is desirable to place therock sample assembly 114 under pressure while conducting the test as itis desirable to simulate actual downhole pressures to witness thecharacteristics of the rock sample assembly 114 under estimated downholeconditions. Once the desired pressure is reached, the analysis may beginusing a CT scan or other measurement scanning techniques using the testmeasurement device. For a CT scan, the radiation are typically X rays,while the detector is typically a scintillator array or even a singlecrystal. The controller and measurement recording device 190 istypically a computer system with motor controllers and switches. In anillustrative embodiment, the X rays are created from electronsaccelerated with voltages ranging from 20-100 kV with power output about10 W.

In another embodiment, there may be a bottom threaded hole with a screwplug (not shown) at the base 172 of the vessel body 160. Note that thepreferred placement of radiation source 180A and detector 180B is asclose as practical to the sample 210. In practice, those locations, whenthe plastic enforcer tube 118 is not present, are where the plasticenforcer tube 118 is shown in FIG. 1. When the plastic enforcer tube 118is present, those locations are approximately 1 mm from the plasticenforcer tube 118. Those of skill in the art will appreciate thelocation being close to but distant enough to avoid complications withbeing right at the surface of the plastic enforcer tube 118. Note thatsubstantially incompressible herein means anything less than 0.2% atoperating pressure. All embodiments or illustrative examples not givenas alternatives to each other are combined with each other as additionaldisclosed embodiments.

What is claimed is:
 1. A micro-CT sample holder for placing a sampleunder hydrostatic pressure during scanning, the sample holdercomprising: a pressure vessel having a first wall region, a second wallregion, and an interior chamber with a generally cylindrical shape witha top opening, wherein included inside the interior chamber are one ormore fixed reference stops; a position-locating anvil for holding thesample, wherein upon inserting the position-locating anvil in theinterior chamber, the position-locating anvil rests on the one or morefixed reference stops, wherein the position-locating anvil is comprisedof a substantially incompressible material; wherein the second wallregion surrounding the location of the sample when inside the pressurevessel is thinner than the first wall region yet sufficient formaintaining a design pressure within the pressure vessel; wherein thepressure vessel includes threads near the top opening; and a compressionscrew assembly for closing the top opening and applying a variablepressure on the pressure vessel by coupling with the threads of thepressure vessel, wherein the compression screw assembly includes apiston acting on a fluid inside the interior chamber.
 2. The sampleholder of claim 1, further comprising a bottom plug sized to fit in theinterior chamber below the fixed reference stops while entrapping acompressible fluid that is compressed by motion of the piston.
 3. Thesample holder of claim 1, wherein the compression screw assembly furthercomprises a pressure gauge.
 4. The sample holder of claim 1, wherein thesample holder has sufficient axial symmetry to rotate through 360degrees, and wherein the indentions further extend around the pressurevessel through all 360 degrees.
 5. The sample holder of claim 4, furthercomprising a plastic enforcer tube positioned to provide additionalstructural support for the pressure vessel across the indentions.
 6. Amicro-CT scanning system, comprising: a micro-CT sample holder placing asample under hydrostatic pressure during scanning, the sample holdercomprising: a pressure vessel having a first wall region, a second wallregion, and an interior chamber with a generally cylindrical shape witha top opening, wherein included inside the interior chamber are one ormore fixed reference stops; a position-locating anvil for holding thesample, wherein upon inserting the position-locating anvil in theinterior chamber, the position-locating anvil rests on the one or morefixed reference stops, wherein the position-locating anvil is comprisedof a substantially incompressible material; wherein the second wallregion surrounding the location of the sample when inside the pressurevessel is thinner than the first wall region yet sufficient formaintaining a design pressure within the pressure vessel; wherein thepressure vessel includes threads near the top opening; and a compressionscrew assembly for closing the top opening and applying a variablepressure on the pressure vessel by coupling with the threads of thepressure vessel, wherein the compression screw assembly includes apiston acting on a fluid inside the interior chamber; and a radiationsource and a radiation detector located on opposite sides of thepressure vessel at the indentions, wherein the radiation source and theradiation detector are coupled to a control module and a data acquitionunit for recording a response of the radiation detector to radiationpassing through the sample.
 7. The micro-CT scanning system of claim 6,wherein the sample holder further comprises a bottom plug sized to fitin the interior chamber below the fixed reference stops while entrappinga compressible fluid that is compressed by motion of the piston.
 8. Themicro-CT scanning system of claim 6, wherein the compression screwassembly further comprises a pressure gauge.
 9. The micro-CT scanningsystem of claim 6, wherein the sample holder has sufficient axialsymmetry to rotate through 360 degrees, and wherein the indentionsfurther extend around the pressure vessel through all 360 degrees. 10.The micro-CT scanning system of claim 6, wherein the sample holderfurther comprises a plastic enforcer tube positioned to provideadditional structural support for the pressure vessel across theindentions.
 11. A rock sample inspection method, comprising: preparing asample of formation rock by encapsulating the sample; inserting thesample into a vessel body as part of a test assembly; enclosing thesample within an low compressibility fluid; applying pressure to aninterior of the vessel body by tightening a compression screw employinga piston acting on said low compressibility fluid; monitoring thepressure; conducting a test on the sample; and recording results of thetest for further analysis.
 12. The method of claim 11, furthercomprising: prior to said conducting the test on the sample, allowingthe vessel body and fluid to reach temperature equilibrium.
 13. Themethod of claim 11, further comprising: prior to said applying pressureto the interior of the vessel body, conducting the test on the samplewithout additional applied pressure.